The ordinary conventional flush toilet has a rim of rectangular cross-section formed at the upper edge portion. The interior of the rim is used as a water channel and the undersurface of the rim is formed with holes or slits for discharging cleansing water onto the waste receiving surface.
However, this type of flush toilet has a problem in that the boundary region between the rim undersurface and the waste receiving surface is a blind area that cannot be seen from above. Dirty matter is therefore liable to remain at this region, particularly on the rim undersurface, which is not reached by the cleansing water vortex. Moreover, the boundary region sometimes cannot be thoroughly glazed owing to its concealed location. This also leads to adherence of dirty matter.
In order to overcome this problem of the ordinary conventional flush toilet of this type, the assignee previously developed a new flush toilet structure that does not use the interior of the rim as a water channel. This structure is described in WO98/16696 (Ref. No. 1).
In this flush toilet, the inner surface of the rim facing the bowl and the waste receiving surface are made continuously smooth so as not to form a blind area and cleansing water is supplied to the bowl from a single water spout provided at the rear of the bowl. The cleansing water is jetted from the water spout in the vicinity of the boundary region between rim and the waste receiving surface to form a vortex that carries the cleansing water over the entire waste receiving surface.
In the flush toilet of Ref. No. 1, the rim inner surface is given an overhang configuration to prevent cleansing water from overflowing to the outside of the toilet bowl. Consideration is also given to the fact that causing the cleansing water to complete a full circle is not practical because the long distance involved would necessitate a high water discharge pressure. In addition, the need to spread the cleansing water uniformly throughout would restrict freedom of shape selection. The proposed structure therefore calls for the provision of left and right facing water spouts at the rear of the bowl so that cleansing water from the water source can be supplied dividedly to the left and right water spouts.
Japanese Patent Laid-Open Publication No. 9-125502 (Ref. No. 2) teaches a flush toilet in which a distributor is installed at the bottom of a low tank and bubbly water (cleansing water) is dividedly supplied to the bowl in opposite lateral directions.
In addition, Japanese Patent Laid-Open Publication No. 2000-96689 (Ref No. 3) teaches a flush toilet equipped with multiple water spouts used to form a single vortex. Specifically, water spouts are provided at two locations at the front and rear ends of the bowl and at four locations midway between the front and rear ends of the bowl on either lateral side.
A flush toilet using a cleansing water tank unavoidably experiences a decline in waterhead during supply of cleansing water from the tank to the toilet because the amount of water in the tank decreases as flushing proceeds. When an attempt is made to secure a vortex sufficient for cleansing by increasing the waterhead, the amount of water discharged from the individual water spouts at the start of flushing increases to the point that the cleansing water is liable to spew out of the bowl. Moreover, cleansing of the bowl is liable to be inadequate because at the final stage of flushing the cleansing water vortex does not reach the extremities sufficiently.
In a direct-connection flush toilet, i.e., a toilet supplied with cleansing water directly from a service water pipe, variation in the amount of water supplied during flushing can be reduced when the toilet is flushed by a cleansing water vortex because the flow of cleansing water from the service water pipe is made constant by a constant flow valve or the like. When the pressure of the service water supplied to the water spout(s) is low, however, the cleansing water vortex becomes insufficient to make thorough flushing of the toilet impossible.
Although this problem can be solved by providing a plurality of water spouts, the structures taught by Ref. No. 1 and Ref No. 2 of using a distributor or the like to spout water in opposite lateral directions is disadvantageous in the point that the two oppositely directed vortices collide at the center region of the toilet bowl to cause splashing and also in the point that a smooth siphon effect is inhibited.
Although Ref No. 3 teaches a structure that uses multiple water spouts to form a vortex in a single direction, mere provision of multiple water spouts does not enable the flushing that the bowl (bowl surface) of a flush toilet requires to convey and discharge waste efficiently. Namely, it involves problems from the practical aspect in that it makes no suggestion whatsoever regarding specific structural features for achieving these requirements, such as the location of the water spouts and method of supplying cleansing water to the water spouts.
On the other hand, water-conserving type flush toilets with a tank capacity of 6 to 8 liters have been moving into a position of dominance in recent years. Since the amount of cleansing water supplied is less than that from the conventional tank, waste must be discharged by producing a siphon effect shortly after the start of water supply. If the siphon effect should come into action later, the waste discharge performance will decrease in proportion to the delay. The siphon jet system that supplies cleansing water directly from jet holes on opposite sides of the mouth of the discharge passage is preferably adopted in order to ensure production of a siphoning action at an early stage. In the case of the siphon jet flush toilet, however, a larger amount of water has to be supplied to the jet holes than to the water spouts that form the vortex. As a result, cleansing of the bowl (surface) becomes insufficient because not enough water is supplied to the water spouts, thus posing a fundamental problem. From the foregoing it will be understood that bowl surface cleansing performance and waste discharge performance are both very basic requirements that must be met in a flush toilet. A flush toilet equipped with a water-conserving tank that can overcome the foregoing problems is therefore strongly desired.